The present invention relates generally to electromagnetic bandgap materials for isolating antennas. More particularly, the present invention relates to narrow reactive edge treatments and methods for manufacturing the same. One embodiment of the invention is a surface treatment that may be applied to laptop computers or other wireless devices.
In many applications, two or more adjacent antennas may couple energy in an undesirable fashion. The coupling reduces the efficiency of all antennas involved and may drastically limit the range and reliability of radio devices using the antennas.
One particular application which requires multiple antennas is a laptop computer with Bluetooth and wireless local area network (WLAN) capabilities. Bluetooth is a wireless data communication standard operating at approximately 2.4 GHz with a range of approximately 10 meters. WLAN data standards include a group of standards propounded by the Institute of Electrical and Electronics Engineers (IEEE) and generally called 802.11. These include IEEE standard 802.11b, also operating at 2.4 GHz. Both Bluetooth and WLAN standards such as 802.11b allow high-speed data communication for mobile device such as laptop computers.
Both Bluetooth and WLAN standards such as 802.11b allow high-speed data communication for mobile devices such as laptop computers. Many such devices will be equipped with transceivers and antennas for both technologies. Electrical standards are under development to define the electrical interoperation of these radio devices. The required minimum isolation between antennas for simultaneous operation of Bluetooth and 802.11b WLAN radios is generally acknowledged to be between 30 dB and 40 dB. Untreated antennas typically exhibit 15 dB to 25 dB of isolation when installed on a laptop.
FIG. 1 illustrates coupling of energy between antennas mounted on a mobile device 100. In the example of FIG. 1, a first antenna 102 and a second antenna 104 are mounted at the periphery 106 of the metal housing 108 of the display portion 110 of a laptop computer. The laptop computer also includes a base portion (not shown) to which the display portion is mounted, the base portion typically including a case containing a motherboard, keyboard and other conventional laptop components.
The conductive metal housing or chassis 108 provides a surface where electric fields can attach. FIG. 1 illustrates vertical electric field lines 112. Energy is propagated from one antenna 102, 104 to the other antenna 104, 102 through waves set up by the electric fields represented by the electric field lines 112. Energy can be propagated in both directions, from the first antenna 102 to the second antenna 104 and from the second antenna 104 to the first antenna 102. The effect is to increase mutual coupling between antennas.
Surface treatments have been developed to promote isolation between antennas such as the antennas 102, 104. A first example surface treatment is made of magnetic radar absorbing material (MAGRAM). This is typically an elastomeric material such as rubber or silicon or urethane that has been loaded with small magnetic particles such as carbonyl iron or ferrite powers. The drawbacks with this solution include the mass of the MAGRAM material. The surface treatments are relatively heavy even for thin MAGRAM, typically 1 to 3 pounds per square foot for thicknesses of 0.062 inches to 0.20 inches. Also, the MAGRAM absorbs radio frequency (RF) energy rather than re-directing the energy. This will degrade antenna efficiency when placed within the antennas near field.
Additional surface treatments that are capable of suppression of transverse magnetic (TM) mode surface waves include carbon loaded foam and semi-conductive honeycomb core materials. However, both of these classes of materials require a relatively thick absorber to be effective, often one-quarter to one-half of a free-space wavelength in thickness. Also, as with the MAGRAM material, these materials are RF absorbers that will degrade antenna efficiency when used in the near field of an antenna.
Accordingly, there is a need for an improved edge treatment for isolating two or more antennas, particularly on a mobile device such as a laptop computer. What is needed is a surface treatment that does not absorb radio frequency energy, but re-directs energy away from the treated surface, is relatively low profile and light weight for mobile applications, and can be mass produced using mature manufacturing processes.